Phosphate rocks contain calcium phosphate minerals largely in the form of apatite, usually together with other minerals, e.g. silicate minerals and carbonate minerals, such as calcite. Apatite is a generic name for a group of calcium phosphate minerals also containing other elements or radicals, such as fluorapatite, chlorapatite, hydroxylapatite, carbonate-rich fluorapatite and carbonate-rich hydroxylapatite.
It is well-known to separate the valuable phosphate minerals from the gangue by using a froth flotation process where the phosphate minerals are enriched in the float.
Good performance in a froth flotation process is achieved by a combination of, on the one hand, a good separation of the valuable mineral from the gangue by using a selective collector and, on the other hand, the froth characteristics. The froth characteristics include both the height and the stability of the froth. It is important in the flotation process that the froth collapses as soon as possible after the air supply is stopped, since this is directly connected to the flotation performance. A too stable froth will cause both entrainment of particles and froth product pumping problems. Entrainment, especially on a large scale, will result in decreased selectivity (grade, recovery). Problems with froth product pumping will make a process of flotation technically impossible.
Collector performance may be improved by using collector combinations of a primary (main) collector and a secondary collector (co-collector). In this document the term “collector composition” shall be used to describe compositions containing both a primary and a secondary collector.
For many decades secondary collectors have been used together with primary ionic collectors in salt-type mineral flotation to improve the performance of the primary collector. Nonylphenol ethoxylates have been the dominating nonionic surfactant used as a co-collector in a combination with sarcosine-type primary collectors in selective flotation of apatite from calcite-containing ores.
SE 409291 discloses a method for foam flotation of calcium phosphate-containing minerals, using an amphoteric surface-active compound as the primary collector. The primary collector's flotating ability may further be strengthened by the presence of a secondary collector, which is described as a polar, water-insoluble, hydrophobic substance having affinity to the mineral particles that have been coated by the primary collector. Examples of the polar components are e.g. water-insoluble soaps, such as calcium soaps, water-insoluble surface-active alkylene oxide adducts, organic phosphate compounds, such as tributyl phosphate, and esters of carbonic acids, such as tributyl ester of nitrilotriacetic acid. In the working examples nonylphenol that has been reacted with two moles of ethylene oxide was used as the secondary collector.
The secondary collector disclosed in SE'291 still is considered a good choice in treating ores, as it provides for an excellent mineral recovery at a P2O5 grade of higher than 30%. However, due to environmental concerns, an intense search for a replacement of nonylphenol ethoxylates has been ongoing for a long time.
EP 0 270 933 A2 discloses mixtures as collectors for flotation of non-sulfidic ores that contain an alkyl or alkenyl polyethylene glycol ether that is end capped with a hydrophobic group and an anionic tenside. The end capped alkyl or alkenyl polyethylene glycol ether in embodiments is based on a fatty alcohol, preferably a C12 to C18 fatty alcohol. In comparative Examples in EP 0 270 933 also non-end-capped fatty alcohols are used together with anionic tensides. In EP 0 270 933 no disclosure is made of using fatty alcohols having a degree of branching of 1 to 3, and the molecules exemplified in the document, though environmentally more friendly than nonylphenol ethoxylates, do not perform as well as these nonylphenol ethoxylates as collectors for flotation of non-sulfidic ores in terms of mineral recovery at the desired high grades.
Thus, there is still a need for secondary collectors having a better environmental profile than nonylphenol ethoxylates that perform equally well.